The tospoviruses are a group of negative-strand RNA viruses, which form a separate genus within the arthropod-borne family of Bunyaviridae and are unique within this family with respect to their ability to infect plants. Based on serological differences and sequence divergence of the nucleoprotein gene, four different tospovirus species have so far been established: tomato spotted wilt virus (TSWV), tomato chlorotic spot virus (TCSV), groundnut ringspot virus (GRSV), and impatiens necrotic spot virus (INSV). Based on serological data, groundnut bud necrosis virus, watermelon silver mottle virus and groundnut yellow spot virus have been proposed as additional members of the Tospovirus genus.
Tospoviruses are the only plant viruses that are transmitted by thrips species in a propagative manner. The type species of the genus Tospovirus, TSWV, has a very broad host range, encompassing more than 650 plant species within 70 families, including many important crops and ornamentals. The TSWV particle consists of a nucleocapsid core, in which the genomic RNAs are tightly associated with the nucleoprotein (N), surrounded by a lipid membrane containing two types of glycoprotein-protrusions G1 and G2. In addition, several copies of the putative viral RNA-dependent RNA polymerase are present in the virus particle.
The complete nucleotide sequence of the three genomic RNAs of TSWV has been determined. The L RNA is of complete negative polarity and encodes the putative viral polymerase of 331.5 kilodalton (ID). The M and S RNAs both have an ambisense coding arrangement and are translated from subgenomic messenger RNAs. The M RNA codes for the precursor of the membrane glycoproteins G1 and G2 (of 78 kD and 58 kD respectively) and a non-structural protein (NS.sub.M) of 33.6 kD, which represents the putative viral movement protein. The S RNA codes for the N protein of 28.8 kD and another non-structural protein (NS.sub.S) of 52.4 kD.
When compared to coding arrangements of the genomic RNAs of other members of the family Bunyaviridae, tospoviruses are unique in having an ambisense M RNA segment. The additional presence of the NS.sub.M gene on the viral strand seems to be an evolutionary adaptation of Bunyaviridae to the plant kingdom. It has been proposed that the NS.sub.M gene product enables the virus to pass the cell wall boundary, suggesting NS.sub.M to represent the viral cell-to-cell movement protein.
Engineered resistance to tomato spotted wilt tospovirus (TSWV) has been accomplished previously by expressing the viral nucleoprotein (N) gene in transgenic tobacco. Recently, engineered TSWV resistance has been introduced in tomato plants and tomato hybrids. Similar levels of protection, i.e. complete immunity to the virus in homozygous S2 plants, have been observed when an untranslatable N gene was expressed, indicating that the N gene-based resistance is, at least for a major part, RNA-mediated.
Besides tospoviral nucleoprotein and positive strand RNA virus coat protein sequences, other, non-structural gene sequences have been used to confer engineered virus resistance, including replicase and protease genes.